In digital data storage, for protection of payload data against errors, error correction encoding is applied before channel coding on the recording side, and correspondingly error correction decoding is performed after channel decoding on the readout side. So-called block codes are one widely used class of error correction codes. Reed-Solomon codes are one example of block codes.
Besides random errors, optical storage channels usually suffer from long bursts of errors induced by scratches, dust, fingerprints, air bubbles and other defects. To combat burst errors, interleaved Reed-Solomon codes are widely used. By the interleaving step, a burst error afflicting a long row of contiguous symbols is transformed into single symbol errors spaced apart widely enough to be correctable by a subsequent Reed-Solomon code.
It is well known that the error correction capability of block codes can be doubled if locations of erroneous symbols are known. Hence an important sub-task of practical error correcting schemes is to estimate or detect these error locations also known as “erasures”.
For erasure detection in a burst error environment, a so-called “picket code” scheme has been proposed in U.S. Pat. No. 6,378,100, where user data are regularly interspersed with so-called “burst indicating subcode” or BIS fields. This approach assumes and aims at data sequences containing bursts of errors. Hence, whenever any two consecutive BIS fields are diagnosed as erroneous, the user data between them are assumed to be an erasure. The arrangement of BIS fields and user data is illustrated in FIG. 1. The BIS fields within each sector are grouped together into a so-called BIS codeword, which in turn is provided with a very strong error correction code of its own.
This known scheme of erasure detection may be seen to have the following drawbacks:                Erroneousness of the BIS fields can not be detected individually, it is always complete BIS codewords that have to be read and decoded. Consequently, the erasure detection scheme based on the BIS fields has a latency or delay of one sector.        Inserting the BIS fields plus their error protection causes a considerable redundancy overhead.        
Another approach of erasure information generation is disclosed in “A New Error Correction Algorithm for High-Density Optical Storage Systems”, JJAP-43-4867. There, rows are divided into blocks. An inner (i+1, i, 2) RS-Code, which is only capable of detecting errors, but not of correcting them, is applied to i evenly distributed information bytes of each block. The resulting RS-Code parity byte is appended to each block. Syndrome checking is used to check the inner RS-codeword integrity. Blocks with inner RS-codeword failure are declared to be erased. The approach abandons wordwise interleaving between different rows. Instead the symbols of each inner RS-codeword are equally distributed along the corresponding block within a row. No error locating RS code is necessary since the physical dilation of inner RS-codewords is small. The absence of interleaving reduces latency. However, the approach of JJAP-43-4867 can be seen to have the drawback that additional redundancy is needed for erasure generation.
An improved erasure detection scheme is therefore desirable.